1. Field of the Invention
Aspects of the present invention relate to a composition containing a proton-conductive copolymer, a polymer electrolyte membrane formed from the composition, a method of producing the membrane, and a fuel cell using the membrane.
2. Description of the Related Art
Fuel cells can be classified, according to the type of electrolyte used, as polymer electrolyte membrane fuel cells (PEMFC), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), and the like. The operating temperature of the fuel cell and the material used in the constituent elements will vary, according to the electrolyte used.
Fuel cells can also be classified according to a method of supplying fuel to the anode. Thus, fuel cells can be divided into an external reforming type, in which fuel is converted to hydrogen-enriched gas by a fuel reformer and then supplied to an anode, and a direct fuel feeding type, or an internal reforming type, in which fuel is supplied directly to an anode in a gaseous or liquid state.
A representative example of a direct fuel feeding fuel cell is a direct methanol fuel cell (DMFC). In general, the direct methanol fuel cell uses an aqueous solution of methanol as fuel, and a proton-conductive, polymer electrolyte membrane as an electrolyte. In this light, a DMFC is a type of PEMFC.
A PEMFC can provide a high output density while having a small size and light weight. Moreover, a PEMFC can lead to a simplified configuration for a power generating system. A PEMFC typically comprises an anode (fuel electrode), a cathode (oxidant electrode), and a polymer electrolyte membrane disposed between the anode and the cathode. The anode of the PEMFC includes a catalyst layer to promote oxidation of fuel, and the cathode of the PEMFC includes a catalyst layer to promote reduction of an oxidant.
The fuel that is supplied to the anode of a PEMFC is typically hydrogen, a hydrogen-containing gas, a vapor mixture of steam and methanol, an aqueous solution methanol, or the like. The oxidant that is supplied to the cathode of a PEMFC is typically oxygen, an oxygen-containing gas, or air.
At the anode of the PEMFC, fuel is oxidized to generate protons and electrons. The generated protons are transferred to the cathode through the electrolyte membrane, and the electrons are transferred to an external circuit (load) through a conducting wire (or a collector). Then, the protons are transferred through the electrolyte membrane to the cathode of the PEMFC, the electrons are transferred from the external circuit through the conducting wire (or a collector), and combined with oxygen to form water. Here, the flow of electrons passing through the anode, the external circuit, and the cathode is referred to as electricity or electric current.
In the PEMFC, the polymer electrolyte membrane serves not only as an ion conductor for the transfer of protons from the anode to the cathode, but also as a separator to block mechanical contact of the anode and the cathode. Thus, the properties required from a polymer electrolyte membrane include excellent ion conductivity, electrochemical stability, high mechanical strength, thermal stability at operating temperatures, and the ability to be easily formed into a thin film.
The material generally used for forming a polymer electrolyte membrane is exemplified by a polymer electrolyte, such as, a sulfonate perfluorinated polymer (e.g., NAFION of DuPont Corporation) having a backbone composed of fluorinate alkylenes and side chains composed of fluorinated vinyl ether with sulfonic acid groups at the ends of the chains. Such polymer electrolyte membrane holds an adequate amount of water, thus exhibiting excellent ion conductivity.
However, such a polymer electrolyte membrane does not have a satisfactory methanol permeability, and is costly to produce. Furthermore, when the operating temperature is above 100° C., the ion conductivity is severely degraded due to the evaporation of water from the membrane, and the electrolytic function of the membrane is impaired. It is therefore virtually impossible to operate a PEMFC employing such a polymer electrolyte membrane at normal pressure and at a temperature exceeding 100° C. Thus, conventional PEMFCs have been operated mainly at a temperature of not higher than 100° C., for example, but not limited thereto, about 80° C.
Furthermore, in general, when the ion conductivity of an electrolyte membrane is increased, the water permeability thereof is also increased. This property is usually associated with an increase in the methanol permeability. Thus, it is difficult to for an electrolyte membrane to simultaneously have a high ion conductivity and a low methanol permeability. When a comparison is made between the amounts of water and methanol that have passed through a potential electrolyte membrane, and the amounts of water and methanol that pass through a standard electrolyte membrane (e.g., NAFION 115), the potential electrolyte membrane may be determined to be useful as a DMFC electrolyte membrane, if the comparison ratio for the amount of passed water is greater than or equal to 1, while the comparison ratio for the amount of passed methanol is less than 1.
Extensive research is being conducted to create a polymer electrolyte membrane which can overcome the aforementioned problems, and can be used as a substitute for a NAFION electrolyte membrane. As the material forming such polymer electrolyte membrane, there is known a block copolymer comprising a hydrocarbon-based repeating unit, such as, a styrene repeating unit, an ethylene-r-butylene repeating unit, or an isobutylene repeating unit.
However, such block copolymers have problems in that the methanol crossover and swelling of the block copolymers are so extensive that the dimensional stability of a membrane-electrolyte assembly (MEA) formed therewith, is very poor.
Thus, the present invention seeks to address at least the problems of conventional polymer electrolyte membranes as described above.